Kafka connector: date type cannot use string in JSON

**Bug Description**
+ version: v0.6.0
Date type cannot use string in JSON, but timestamp should be used. For example, 1659512628000 can be used, but "2022-08-03" cannot be used

**Expected Behavior**
Compatible string types

**Steps to Reproduce**

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Look for the Kafka connector code in the OpenMLDB source, likely under connectors/ or similar. Examine how date types are parsed from JSON input. The issue is that string date formats like "2022-08-03" are not accepted, only numeric timestamps. Find the relevant parsing logic and extend it to handle common string date formats. Testing will involve setting up a Kafka connector test with JSON containing string dates.

Scritto dal modello di indicizzazione a partire dal testo della issue.